New Study Shows Multi-Fault Earthquakes Likely in Seattle Region of Washington State

Thursday - September 28 2023, 15:30 UTC - 1 year ago

A new study reveals that over 1,100 years ago, an earthquake cluster in the Puget Sound region led to landslides, upliftment of fault scraps, and a local tsunami. The study also suggests the potential for a single multi-fault earthquake with a magnitude of around 7.8, and emphasizes the need to update current seismic hazard models to account for such scenarios.

Fault zones are areas in a city or a state more prone to earthquakes. They have active faults that may have been created many years ago and have the potential for surface rupture. As per a study published today (27 September), there is an urgent need to update local hazard estimates, especially in the Seattle area of Washington state, to account for the possibility of much more devastating "multi-fault" earthquakes.

The Puget Sound region, comprising major metropolitan areas such as Seattle and Tacoma, contains seismically active geologic faults. It is a structurally complex, seismically active trough between the Coast Range and the Cascade volcanic arc.

According to the Seattle municipality, deep quakes are the most common large earthquakes that occur in the Puget Sound region. Over the years, deep quakes of magnitude larger than six have occurred in 1909, 1939, 1946, 1949, 1965, and 2001.

The study says that over 1,100 years ago, an earthquake cluster in the Puget Sound region led to landslides, upliftment of fault scraps, and a local tsunami, as a result of which a lot of local trees were drowned and buried across an area of 193 square miles (500 square kilometers).

It’s all related .

An earlier dendrochronological study of these trees established that a tree that fell into Lake Washington in a landslide had the same timeline as the tree that was buried in a tsunami along the Puget Sound shoreline. This confirmed the widespread impact of an earthquake 1,100 years ago but did not establish an exact calendar year of the event or linkages with events on adjacent faults in far-off places.

"In this study, we apply dendrochronological dating techniques to precisely link dates of tree death across six sites that span the Seattle and Saddle Mountain fault zones (Puget Sound region) and anchor these earthquake-killed tree-ring chronologies with an absolutely dated chronology developed from live-collected trees," said the authors in the study.

The team hired divers to bring to the surface a tree sample that they collected from an earthquake-downed tree in Washington's Price Lake.

The team used radiocarbon and tree-ring dating to determine when the trees died and found the trees had died between 923 and 924 AD, within a six-month period, which indicated that the earthquakes across the Puget Sound region took place in a short period of time.

The researchers estimate that a single multi-fault earthquake could have had a magnitude of around 7.8, while two earthquakes within that six-month period would each have exceeded 7.3, said the press release. "These scenarios, which are not recognized in current hazard models, increase the maximum earthquake size needed for seismic preparedness and engineering design," wrote the researchers in the study.

The study has been published in the journal Science Advances.

Study abstract: .

Compound earthquakes involving simultaneous ruptures along multiple faults often define a region’s upper threshold of maximum magnitude. Yet the potential for linked faulting remains poorly understood given the infrequency of such events in the historic era. Geological records provide longer-term insights into extreme earthquakes, but analytical challenges arise from difficulties in connecting events across many tens of kilometers. Here, we use a dendrochronological dating approach to establish a timescale for linked earthquakes across the Seattle and Saddle Mountain fault zones (~193 km2) in the Puget Sound region of Washington, USA, over 1,100 years ago. We used radiocarbon and tree-ring dates to resolve a precise timeline for six earthquake-killed trees, which collectively assert a 6-month long cluster of earthquakes between AD 923 and 924. Applying a multi-fault rupture model to the spatial distribution of the dated trees implies that two ∼M7.3 or greater earthquakes, or a single M7.8 or greater earthquake, occurred along this fault network. Such scenarios, not accounted for in contemporary hazard models, increase the maximum earthquake size needed for seismic preparedness and engineering design.